Showing papers by "Tongji University published in 1998"

Thermal degradation of cellulose and cellulose esters

Abstract: Cellulose, cellulose diacetate (CDA), cellulose triacetate (CTA), cellulose nitrate (CN), and cellulose phosphate (CP) were subjected to dynamic thermogravimetry in nitrogen and air. The thermostability of the cellulose and its esters was estimated, taking into account the values of initial thermal degradation temperature Td, the temperature at the maximum degradation rate Tdm, and char yield at 400°C. The results show that these polymers may be arranged in the following order of increasing thermostability: CN < CP < regenerated cellulose < filter cotton < CDA < CTA. The activation energy (E), order (n), and frequency factor (Z) of their degradation reactions were obtained following the Friedman, Chang, Coats–Redfern, Freeman–Carroll, and Kissinger methods. The dependence of Td, Tdm, E, n, Ln Z, and char yield at 400°C on molecular weight and test atmosphere is also discussed. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:293–304, 1998

High strength SiO2 aerogel insulation

Abstract: Silica aerogels with titanium dioxide powder and inorganic binders were made using tetraethoxysilane (TEOS), water and HF catalyst, followed by supercritical drying with ethanol leading to crack-free monolithic aerogels. Strength of the monolithic silica aerogels increased from 1.8×104 Pa to 1.2×105 Pa with doping, while the thermal conductivity of the doped silica aerogel remains below 0.02 w/m K at room temperature in air.

Kinetics of thermal degradation of thermotropic poly(p-oxybenzoate-co-ethylene terephthalate) by single heating rate methods

Abstract: The kinetics of decomposition of thermotropic liquid crystalline poly(p-oxybenzoate-co-ethylene terephthalate) (poly(B-co-E), BE polymer) with different monomer ratios in both nitrogen and air were studied by dynamic thermogravimetry (TG) from ambient temperature to 800°C. The kinetic parameters, including the activation energy E′, the reaction order n, and the pre-exponential factor Z, of the degradation of the BE polymers were evaluated by the single heating rate methods of Friedman, Freeman–Carroll and Chang. The BE polymers which degraded in two distinct stages in nitrogen and air, were stable under nitrogen, while almost completely burned in air. The weight losses in the first stage in nitrogen and air were dominated by the thermal degradation of both B and E segments, but the weight losses in the second stage were governed by the thermal degradation of B segment in nitrogen and by the oxidative degradation of both B and E segments in air. The maximum rate of weight loss increased linearly with the increase of E content and heating rate, but as E content increased, the char yield at 800°C in nitrogen decreased linearly. The E′, n and ln Z values of the BE polymers in the first stage of thermal pyrolysis are higher in nitrogen than in air, indicating that the degradation rate is slower in an inert atmosphere. The E′ value increased with increasing heating rate but varied irregularly with the variations of B/E ratios and molecular weight. The n and ln Z values for the BE polymers in nitrogen were found to be in the wide ranges 1·5–5·6 and 12–48min-1, respectively, suggesting a complex degradation process. The estimated lifetimes of the BE polymers at 250°C were calculated to be at least 33 days in nitrogen and 3h in air. © 1998 SCI.

Dynamic analysis on structures base‐isolated by a ball system with restoring property

Abstract: A computational model with its analysis method for base-isolated structures by a ball system with restoring property under seismic force is proposed in this paper, and the programs using numerical integration method and incremental harmonic balance method are developed. The analysis method is verified by shaking table test results of a three-storey masonry model. With these programs, the effect of some factors on the aseismic behaviour of base isolation system are analysed, and the comparison of seismic response between structures with and without base isolation is made. Results of both theory and test show that the above-mentioned base isolation system has apparent advantages over the traditional aseismic structures. © 1998 John Wiley & Sons, Ltd.

Shifts of the Kuroshio axis over the last 20 000 years

Abstract: Core 255 from the southern Okinawa Trough was selected to monitor the shift of the Kuroshio axis over the last 20 000 years During the last glacial maximum the Kuroshio axis had ever shifted outside the Okinawa Trough It entered the Okinawa Trough again at about 6 500 a BP, leading to abrupt increase of sedimentation rate, distinctly coarser sediment and remarkably increased abundance of foraminiferal indicators of the Kuroshio in the core But, during about 4 000-3 000 a BP the abundance of the Kuroshio indicatorPulleniatina obliquiloculata sharply decreased again, indicating that the Kuroshio axis had ever shifted casterly for a short period or the Kuroshio was weakened and that moment

Study of porous dielectrics as electret materials

Abstract: A special class of nano-porous inorganic dielectrics, the silicon dioxide aerogels, with porosities >70% were prepared using single and two-step sol-gel processing and a supercritical drying method. Samples, pretreated at temperatures /spl les/800/spl deg/C for several hours, and with chemical surface modification by HMDS (hexamethyldisilazane), were subjected to microstructure analysis. Electret properties of these samples, corona charged at temperatures ranging from room temperature to 200/spl deg/C, were studied. Isothermal surface potential decay and thermally stimulated discharge measurements were made. The relationship between the microstructure of these samples and their electret properties was discussed. The electret properties of one of the most important fluoropolymeric electret materials, porous PTFE (polytetrafluoroethylene), also were studied. Our results show that porous dielectrics can be good electret materials.

Kinetics of thermal degradation of liquid‐crystalline aromatic polymers

Abstract: The kinetics of thermal degradation of thermotropic liquid-crystalline aromatic polymers under nitrogen and air was systematically investigated by dynamic thermogravimetry. The thermal degradation temperature and kinetic parameters such as activation energy, decomposition order and frequency factor, have been determined by three methods on the basis of a single heating rate measurement. The effect of polymer molecular structure, end group, testing atmosphere and calculating method on the characteristic temperatures, kinetic parameters, and lifetime have been discussed. The initial decomposition temperature and estimated lifetime for the most thermotropic aromatic polymers are above 420°C and larger than 26 days at 200°C or 2.3 hours at 300°C, respectively. Some theoretical explanations of variation of thermal and kinetic parameters have been proposed.

On pluricanonical maps for threefolds of general type

Abstract: — In this paper we will prove that for a threefold of general type and large volume the second plurigenera is positive and the fifth canonical map is birational. Résumé. — Nous prouvons que pour une variété de dimension 3 de type général et de grand volume le second plurigenre est positif et la cinquième application canonique est birationnelle

Price Limit Performance: Evidence from the Tokyo Stock Exchange

Abstract: Price limit advocates claim that price limits decrease stock price volatility, counter overreaction, and do not interfere with trading activity. Conversely, price limit critics claim that price limits cause higher volatility levels on subsequent days (volatility spillover hypothesis), prevent prices from efficiently reaching their equilibrium level (delayed price discovery hypothesis), and interfere with trading due to limitations imposed by price limits (trading interference hypothesis). Empirical research does not provide conclusive support for either positions. We examine the Tokyo Stock Exchange price limit system to test these hypotheses. Our evidence supports all three hypotheses suggesting that price limits may be ineffective.

Western Pacific in glacial cycles:Seasonality in marginal seas and variabilities of Warm Pool

Abstract: A serics of low-latitude marginal seas, ranging from the southern South China Sea in the north to the Arafura Sea in the south, are located within the Western Pacific Warm Pool. As shown by rnicropaleontological, isotopical and organic geochemical analyses, the sea surface temperatures in the marginal seas at the last glacial maximum were much cooler than those in the open Western Pacific Ocean. The emergence of extensive shelves of the marginal seas at the glacial low sea-level stand and the decrease of surface temperatures in their deeper water parts resulted in a remarkable reduction of the ability of vapor and heat transport to the atmosphere, causing variabilities to the Warm Pool in the glacial cycles. The intensification of winter monsoon at the glacial stages not only led to a decrease of the surface water temperature and hence to an enhanced seasonality, but also carried moisture from the sea to the tropical islands, giving rise to the downward shift of snowline and mountainous vegetation zones there. It may offer a new alternative in solution of the “Tropical Ocean Paleo-temperature Enigma”.

Photoluminescence of fullerenes doped in silica aerogels

Abstract: Fullerenes (C60/C70) were encased in silica aerogels with different routes via sol–gel process followed by supercritical drying. Intense visible light emission was observed from the composite under 488 nm Ar+/laser excitation at room temperature. The luminescent properties are found to be different with the samples prepared via different methods. The luminescent intensity was found to change with the changing of the fullerene content. The intensity and peak position of photoluminescence are also changed by thermal treatment.

Simulation of point defects in high-density luminescent crystals: Oxygen in barium fluoride

Abstract: Barium fluoride is an example of a high-density scintillator for detecting high-energy radiation. In use, its luminescent transmission is seriously degraded by radiation damage. This effect is associated with oxygen, among other impurities. At one time it was suspected that oxygen ${\mathrm{O}}^{\mathrm{\ensuremath{-}}},$ having been dissociated from a defect complex by radiation damage, absorbed some of the luminescent energy of the crystal. This explanation has now been abandoned, and the present work shows quantitatively that it is not correct. Specifically, a detailed study of the optical absorption of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ shows that its excitation energy, split by spin polarization, is $\ensuremath{\sim}50%$ higher than the luminescent frequencies of the crystal. Instead, color centers, such as $F$ centers, have come to be suspected. One origin of the color center is shown here to be the dissociation of a defect complex made up of an ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ion bound to a fluoride vacancy, accompanied by electron transfer from oxygen to vacancy, forming an $F$ center. The study of the optical excitation of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ is used to assess the qualitative and quantitative importance of the main elements of the physical model and computational method in such a simulation. These elements include the ion-size effect of ${\mathrm{Ba}}^{2+}$ ions, spin-polarization effects in ground and excited states, electric quadrupole moment consistency between the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ion and the embedding ${\mathrm{BaF}}_{2}$ crystal, basis set augmentation and optimization in the treatment of a quantum molecular cluster that includes the impurity for both ground and excited states, correlation correction, and projection of excited states onto spin eigenstates.

A quantitative approach to deep-water sedimentation in the South China Sea: Changes since the last glaciation

Abstract: The first attempt is made to evaluate quantitatively the changes of accumulation rates in the South China Sea during the last glaciation and Holocene, based on the data of 72 sediment cores taken from six areas deeper than 100 m. As shown from the calculations, the accumulation rate during the last glaciation is much higher than that during the Holocene. The southern and northern continental slopes are distinguished from other areas by the highest accumulation rates, with different features of sedimentation for different stages: The Glacial-Holocene contrast in accumulation rate of terrigenous material is more distinct in the southern slope, while the contrast in biogenic sedimentation rate is more remarkable in the northern slope.

Variations in late Quaternary upper ocean structure of Okinawa Trough : A nannofossil approach *

Abstract: The relative abundance ofFlorisphueru profunda, a calcareous nannofossil species in lower euphotic zone, affords an opportunity to study variations in upper ocean structure such as thickness of mixed layer. As revealed by the nannofossil analysis in sediments since 20 ka B. P. from three Okinawa Trough cores, the percentage ofF. profunda increased significantly from glacial to Holocene, with a declining event ca 4 ka B. P. in its southern part. Its downcore trend is similar to that ofPulleniatina obliquiloculuta, a tropical planktonic foraminifera1 species which lives in subsurface water below thermocline in the modern ocean. Their similar variations imply an eastward shift of the Kuroshio Current, a reduction in thickness of the mixed layer, a shallowing of thermocline and a possible increase of turbidity in the upper ocean during the glaciation. The eastward shift of the Kuroshio might be repeated again about 4 ka B.P.

A Unified 3-D Homogenization Model of Beam Bundle in Fluid

Abstract: A 3-D homogenization model is developed to predict the overall dynamic property of a beam bundle immersed in an acoustic fluid. It is shown that the existing two models, given by Benner and Schumann (1981) and Hammami and Brochard (1991), respectively, are its 2-D application.

Stepwise paleoceanographic changes during the lastdeglaciation in the southern South China Sea: Records of stable isotope and microfossils

Abstract: Quantitative analyses of microfossils and stable isotopic analysis were carried out for Core SCS-12 in the southwestern slope of the South China Sea (SCS). A high-resolution paleoceanographic record for the last 13 ka was revealed with the AMS14C dates. The southern SCS has experienced stepwise paleoceanographic changes since the last deglaciation. The oxygen isotopic stage 1/2 boundary around 12.05 ka B. P. and the end of the last deglaciation around 7.70 ka B.P. are two rapid change periods (corresponding to the terminationIa and terminationIb, respectively), in between is a slow change period. The authors infer that the sea level stood at - 110 m before the terminationIa, roughly the same as today after the termination IB, and about - 50 m in between. Subsequently, the average winter sea surface temperature and salinity obviously increased while paleo-productivity decreased since 12.05 ka B.P. The early Holocene CaCO3 preservation spike, coupled with a high abundance of pteropoda and CaCO3 content, occurred around 7.70 ka B. P.